PTH and PTHrP are important regulators of calcium homeostasis and bone turnover in many diseases. Many of the effects of PTH, PTHrP, and other GPCR agonists on gene expression are transient. We have recently shown that the primary mechanism(s) responsible for termination of gene regulation following stimulation by PTH or [unreadable]-adrenergic agonists is(are) downstream of receptor desensitization, adenyl cyclase activation, and cAMP degradation. PTH and [unreadable]-adrenergic agonists also stimulate PKA activity and transcription factor phosphorylation in a transient fashion. Thus, it is likely that termination of PKA activity is responsible for termination of downstream events, including transcription factor phosphorylation and gene regulation. When overexpressed, protein kinase inhibitor (PKI) inactivates PKA. We therefore hypothesize that termination of PKA activity by PKI is primarily responsible for termination of gene regulation following stimulation by PTH or [unreadable]-adrenergic agonists. Despite the large number of studies examining purified or overexpressed PKI, little is known about the physiological role of these molecules in cells. Our proposed experiments will be the first to determine whether endogenous levels of PKI are sufficient to reduce PKA activity or downstream gene regulation. Our specific aims are: 1) Test the hypothesis that endogenous PKI interacts with PKA following stimulation by PTH or isoproterenol; 2) Test the hypothesis that endogenous PKI is primarily responsible for termination of both PKA activity and gene regulation following stimulation by PTH or isoproterenol; 3) Test the hypothesis that termination of PTH-induced gene regulation by endogenous PKI is physiologically important. These experiments will substantially increase our understanding of the role of PKI in both PKA signaling and gene regulation in mesenchymal cells as well as the mechanisms responsible for balancing the catabolic effects of PTH. Our findings will also serve as paradigm for regulation of PKA signaling in other cell types. [unreadable] [unreadable]